1. Field of the Invention
The present invention relates generally to combined cycle power plants. More particularly, the present invention relates to a method and apparatus for improving the efficiency of a combined cycle power plant by preheating fuel supplied to a combustion turbine of the power plant.
2. Description of the Prior Art
Combined cycle power plants are well known. Typically, a combined cycle power plant comprises a combustion turbine driving an electrical generator, a heat recovery steam generator (hereinafter "HRSG") receiving exhaust gas from the combustion turbine, and a steam turbine driven by the superheated steam that drives another electrical generator. Also, typically, the HRSG comprises three heat exchanger sections, a superheater section, an evaporator section and an economizer section. The exhaust gas from the combustion turbine flows first over the superheater section, then over the evaporator section, and finally over the economizer section. Condensed water from the steam turbine is supplied as feed water to an inlet of the economizer section. The feed water is heated by the exhaust gas and heated feed water is supplied to the evaporator section which produces saturated steam. The saturated steam is thereafter provided to the superheater section which produces superheated steam, which is provided to the steam turbine. Thus, the steam turbine is interposed in a closed loop connecting the inlet and outlet of the HRSG, i.e., the inlet of the economizer section and the outlet of the superheater section.
The heat recovered from the exhaust gas in the HRSG is limited by the temperature difference between the exhaust gas and the steam evaporation temperature (i.e., the evaporator "pinch" temperature). Although the economizer section helps to improve plant efficiency, the additional heat which can be recovered by the economizer section is limited by the water flow rate necessary to match the steam production in the evaporator section.
The present invention takes advantage of the inventors' recognition that the prevailing exhaust gas temperature allows additional heat recovery to improve plant efficiency.